One odd feature of the USA's electoral system is that while it's extremely difficult to alter the voting system for president and vice president (because of the difficulty of passing a constitutional amendment), the states have wide latitude to change the system for state and local races, as we've seen with California's recent adoption of a runoff system or "Top Two", following adoption in other states, like Louisiana and Washington. With more experimentation taking place at the local level, it would not surprise me if 30 or 40 years in the future, we have a strange patchwork of different voting systems in different states.

One problem with Top Two in particular, however, is the coordination problem. In contrast to "normal" runoff systems, there are no party nominations. Candidates run with party affiliations, but an unlimited number of candidates can run from each party, on their own initiative, with no formal backing from the party. So it's possible that if, for example, too many Democrats run for a particular seat, then the Democratic vote will be spread too thin, and two Republicans will make it to the runoff vote.

However, it seems to me that this coordination problem can be solved with IRV. So how about this for a voting system for state and local offices in the USA?: No primaries and no party nominations, but a single IRV election in which an unlimited number of people from each party can run? The party structures would become even more irrelevant than they are now and reduced to being used only for their brand names, but they're already of declining importance, with the rise of open primaries, modern communication technologies, and outside funding from entities like Super PACs. In another few decades, could we see the parties fade into even more irrelevance, with a voting system like this being adopted by several states?

Can you imagine Presidential ballots with hundreds or even thousands of names? Completely impractical.

Like I said, this is unlikely to happen at the presidential level, because it would require a constitutional amendment. More likely to happen for state and local races.

And I don't think it would lead to hundreds of names being on the ballot. The ballot access rules would be the same as what they are now for primaries, except that you'd have the candidates for each party on a single ballot. How many primaries have hundreds of candidates in them?

Granted, that's still a lot. But it seems cheaper and easier to just have one IRV election with that many names on the ballot than to have a separate runoff election.

Easier for poll workers perhaps, but not voters. To make IRV work, you need to keep the number of choices within reason, either by making the ballot requirements stiffer, or by making a partial IRV system that might still lead to runoffs. (I.e. voters get to number their top five choices. If no one gets a majority, the top five show up in a runoff.)

You're right that ballot access issues could lead to many candidates getting on the ballot, and that would make it a bit complicated for IRV.

In any case, I'm not necessarily predicting that this system will happen as much as I'm pointing out that something like this would be the culmination of some of the trends that we're seeing:

-The party primary system (a relatively new thing in the grand scheme of American political history), Super PACs, and modern information technology which make political organizing easier than it used to be are collectively making the parties less important than they used to be. Politicians are acting more as free agents who use the party brand name, but otherwise don't need it. (At least in terms of how they run for office. Once they actually get in office, it's a different story, since the parties are actually more ideologically sorted now than they were a few decades ago.)

-State and local governments are starting to experiment more with different voting systems, including Top Two, and (at least in some cities, though not yet at the state level) even IRV. In some cases, this experimentation is being done with the explicit goal of diluting one of the side effects of the rise of the party primary system, which is that politicians have become more worried about losing primaries than losing general elections, and can become beholden to a small, unrepresentative primary electorate.

Even while there's a sense that party primaries have created this perverse incentive system that feeds polarization, the idea that "the people should decide" on party nominees rather than letting it be decided by state conventions has become fairly entrenched. There are of course still some states that use party conventions to select nominees, but the trend has been against that.

Given that, some sort of mixed runoff system in which all candidates are thrown together into one contest seems like where things might be headed, though like I said, Top Two has some problems, which means that we're likely to get some variations down the line. In any case, the USA's voting system is constantly evolving, and widespread party primaries in their current form are still relatively new, so I wouldn't be surprised to see the electoral system looking completely different by the middle of this century.

One place the party has remained important is to fill vacancies in office or on the ballot. Many states have no special elections for vacant state offices due to cost and low turnout. The party that held the seat typically makes an appointment to serve for the rest of the term. Without a party all races would have to go to special election, and for states with a short legislative session that would likely mean no representation in a district for that critical period.

Ballot vacancies can occur when a candidate withdraws for any reason, including death. Again, the party generally fills the vacancy. Candidates with a similar set of political views may likely have stayed out of a race where there was already a strong (usually incumbent) candidate to carry those views. Without that ability to fill a vacancy the voters may not find anyone with that set of views which may well represent the majority of voters.

Logged

Triple transit of Jupiter on January 24, 2015 with shadows of Europa, Callisto, and Io appearing from left to right. Image captured from Bate Papo Estelar/Astronomio ao Vivo (Brazil) live feed.

My own personal view on filling legislative vacancies is generally 'Why bother?' If seats were to remain empty when vacated, then voters would have an incentive to pick people who are both healthy enough to last out their term and able to do their job and likely to actually do their job instead of campaigning for another one.